1. Field of the Invention
The present invention relates to a lock-up control system for suitably achieving a direct coupling of input and output elements of a torque converter for a vehicle automatic transmission.
2. Description of Related Art
An automatic transmission, inclusive of a continuously variable transmission (CVT), is generally provided with a torque converter in the power train, for multiplying torque and/or absorbing torque fluctuations. A torque converter conducts power transmission between the input and output elements via an operating fluid, and thus suffers from relatively low transmission efficiency and unsatisfactory fuel consumption. In order to overcome these problems, it became customary to adopt a lock-up type torque converter wherein the input and output elements can be directly coupled by engaging a lock-up clutch in a traveling condition in which torque multiplying function and/or torque-fluctuation absorbing function are not required.
For achieving further improved fuel consumption, it is desirable to broaden a lock-up region by maintaining a torque converter in a lock-up state also during inertial traveling, or coasting, wherein an accelerator pedal is released and the throttle opening degree is maintained at, or near, zero, thereby allowing lock-up of the torque converter down to low load and low vehicle speed conditions. Thus, it is a recent trend that the torque converter is maintained in a lock-up state also during coasting. However, in order to avoid an engine stall when a brake pedal is depressed for sudden braking during coasting on a low friction road, it is desirable to quickly release the lock-up by disengaging the lock-up clutch.
In this connection, if the lock-up of a torque converter during coasting occurs by keeping an engaging capacity of a lock-up clutch at a controlling maximum value, as in the lock-up during a normal traveling, the delay in lock-up release based on the lock-up clutch disengagement is prolonged, thereby failing to avoid the engine stall problem upon wheel locking due to the delay of the lock-up release as a result of sudden braking.
To eliminate such a problem, a countermeasure has been proposed wherein the engaging capacity of a lock-up clutch during coasting in a lock-up region is maintained at a minimum coast lock-up capacity required for lock-up under a steady traveling condition, as disclosed in U.S. Pat. Nos. 5,667,458 and 5,616,099, the disclosures of which are herein incorporated by reference. With such a countermeasure, lock-up is conducted at an engaging capacity that is smaller than the controlling maximum value, so that the response to the lock-up releasing is correspondingly quickened, to thereby avoid the engine stall problem even upon wheel locking due to sudden braking.
Such a conventional coast lock-up control is highly effective insofar as coasting is continued in a drive (D) range where engine braking generally is ineffective. However, the following problems may be caused when engine braking is desired during coasting and a low (L) range is thus selected as an engine-braking range.
FIG. 4 is an operation time-chart showing the coast lock-up of a conventional lock-up control in the case of a changeover from the D range to the L range at a moment t1, during a coasting with the throttle valve opening TVO kept at 0/8 by releasing the accelerator pedal. It is assumed that an engaging capacity of a lock-up clutch, which is the differential pressure (PA-PR) between an apply pressure PA and a release pressure PR of the lock-up clutch, is kept at the required minimum controlled value indicated by a solid line as described in the patent documents cited above. On this occasion, the demanded value of the lock-up clutch engaging capacity indicated by a broken line is suddenly increased after the moment t1 and exceeds the controlled value indicated by the solid line at a moment t2, since the engine braking torque is added to an inertia torque corresponding to the raised amount of an engine revolution speed Ne due to the change-over to the L range.
The lock-up clutch engaging capacity thus tends to become insufficient relative to the demanded value after the moment t2, thereby resulting in undesirable disengagement of the lock-up clutch. Furthermore, once the lock-up clutch has been disengaged, the lock-up clutch cannot be reengaged unless the lock-up clutch engaging capacity is increased to a value higher than the demanded value. As can be noted from a decrease in the engine revolution speed Ne after the moment t2 apart from a turbine revolution speed Nt (torque converter output revolution speed), and also from the temporal transition of a vehicle deceleration, despite the changeover from the D range to L range, an expected magnitude of the vehicle deceleration cannot be obtained, or this may result in a situation wherein the vehicle deceleration is rather decreased.
Namely, in the conventional coast lock-up control, when the D range having an ineffective engine braking function is changed over to the L range as an engine-braking range due to a necessity for an engine-braking during coasting, there may arise such problems that the intended improving effect of the fuel efficiency is not fully achieved since the lock-up of the torque converter is released even in the lock-up region, and/or unnatural driving feel is caused due to unexpectedly low, or occasionally decreasing engine braking force upon the change-over.
It is a primary object of the present invention to provide an improved lock-up control system for a torque converter, capable of suitably conducting the engaging capacity control of a lock-up clutch upon changeover from a non-engine-braking range to an engine-braking range, to thereby avoid the above-mentioned problems of the prior art.
It is another object of the present invention to provide an improved lock-up control system for a torque converter, whereby an appropriate engaging capacity control of the lock-up clutch can be readily conducted.
According to a first aspect of the present invention, there is provided a lock-up control system for a torque converter including a lock-up clutch engageable under a control of an engaging capacity of the lock-up clutch to thereby lock up input and output elements of the torque converter, said lock-up control system being operative during coasting in a lock-up region requiring engagement of the lock-up clutch, for bringing the engaging capacity of the lock-up clutch to a coast lockup capacity smaller than a lock-up capacity under a driving condition at the same vehicle speed, said lock-up control system being further operative when an engine braking range is selected during the coasting in the lock-up region, for bringing the engaging capacity of the lock-up clutch to a value greater than the coast lock-up capacity.
According to a second aspect of the present invention, there is provided a lock-up control system for a torque converter of an automatic transmission, including a lock-up clutch engageable under control of an engaging capacity of the lock-up clutch to thereby lock up input and output elements of the torque converter, said automatic transmission including a range changeover means for effecting changeover between a non-engine-braking range and at least one engine-braking range, said lock-up control system comprising: a lock-up clutch control means being operative during coasting in a lock-up region requiring engagement of the lock-up clutch, for bringing the engaging capacity of the lock-up clutch to a coast lockup capacity smaller than a lock-up capacity under a driving condition at the same vehicle speed; said lock-up control means being further operative when it is judged that said range changeover means is operated in the lock-up region to effect changeover into an engine braking range, for bringing the engaging capacity of the lock-up clutch to a value greater than the coast lock-up capacity.
In the torque converter to which the lock-up control system according to the present invention is applied, the input and output elements of the lock-up clutch are directly coupled to each other in the lock-up region. Furthermore, during coasting in the lock-up region, the engaging capacity of the lock-up clutch is brought to a coast lockup capacity smaller than a lock-up capacity under a driving condition at the same vehicle speed.
With the control system according to the present invention, the engaging capacity of the lock-up clutch is increased to be greater than the coast lock-up capacity, upon selecting an engine braking condition during coasting in the lock-up region. Thus, even when the demanded value of the lock-up clutch engaging capacity is suddenly increased due to the inertia torque of the raised amount of input rotation associated with the range changeover and also due to an engine braking torque, it is possible to maintain sufficient lock-up clutch engaging capacity, thereby avoiding undesirable disengagement of the lock-up clutch.
Therefore, the present invention effectively eliminates such problems upon the range changeover into an engine braking range, such as the lock-up being released even in the lock-up region with the result that the intended effect of improved fuel efficiency is not fully achieved, and/or unnatural driving feel is caused due to unexpectedly low, or occasionally decreasing engine braking force upon the change-over even though an engine braking range has been selected.
With the control system according to the second aspect of the present invention, in particular, since the engaging capacity of the lock-up clutch can be detected highly accurately, further advantages can be achieved in that it is unnecessary to excessively increase the hydraulic pressure of the working fluid and it is thus possible to realize improved fuel consumption.
For positively avoiding an engine stall due to a delay in a lock-up releasing even in the case of a sudden braking, it is preferred that the control system during coasting in a lock-up region brings the engaging capacity of the lock-up clutch to a minimum coast lock-up capacity required for the lock-up under a steady traveling condition.
For bringing the engaging capacity of the lock-up clutch to a value greater than the coast lock-up capacity according to the present invention, it is preferred that the control system upon selection of the engine braking range during coasting in the lock-up region brings the engaging capacity of the lock-up clutch to a controlling maximum value. Such a design makes it possible to positively avoid disengagement of the lock-up clutch even when an engine-braking range is selected in any driving conditions, and to readily conduct an appropriate capacity control of the lock-up clutch according to the present invention, besides that the design itself is highly advantageous from the viewpoint of structural simplification and cost reduction.